Zinc oxide varistors are polycrystalline ceramics which exhibit highly nonlinear current-voltage characteristics. Varistors are used in television sets and other home appliances to protect them from damage due to power surges. However, the most common use is to protect high power transmission lines. Varistors function as insulators below a certain characteristic voltage, termed "switching" or "breakdown" voltage; they allow only small amounts of current to flow. At voltages greater than this characteristic value, the varistor becomes conductive and voltage across the varistor remains at the characteristic value while current flow increases. The varistor properties of ZnO-based ceramics, expressed as J=(E/K).sup..alpha. where J is the current density thru the varistor, E is the field across the varistor, K is a constant, and .alpha. is the nonlinearity coefficient, were first reported by Matsuoka, et al., Jap. J. Appl. Phys. 10(6), 737-46, 1971. The breakdown field is commonly denoted as E.sub.B. Commercialization has led to considerable effort in improving varistor properties such as .alpha. and stability and in obtaining a basic understanding of varistor phenomena.
It is generally believed that the field developed across a varistor is related to insulating properties of grain boundaries and that the voltage drop across a single grain boundary is on the order of 3 volts. It is known that these grain boundary effects can be modified by the presence of chemical additives. The addition of Bi.sub.2 O.sub.3 to polycrystalline ZnO, for example, increases the low voltage resistivity and also acts as a sintering aid, presumably due to the formation of a ZnO-Bi.sub.2 O.sub.3 eutectic [W. G. Morris, "Physical Properties of the Electrical Barriers in Varistors", J. Vac. Sci. Technol. 13(4), 926-31 (1976); J. Wong, "Sintering and Varistor Characteristics of ZnO-Bi.sub.2 O.sub.3 Ceramics", J. Appl. Phys. 51(8), 4453-9 (1980); E. M. Levin and R. S. Roth, "Polymorphism of Bismuth Sesquioxides: II", J. Research Natl. Bur. Stand., Section A, 68(2), 197-206 (1964), which disclosures are incorporated by reference herein.] Since Bi.sub.2 O.sub.3 is essentially insoluble in ZnO, it segregates either at grain boundaries or in second phases at grain boundary junctions: [W. D. Kingery, J. B. VanderSande, and T. Mitamura, "A Scanning Transmission Electron Microscopy Investigation of Grain Boundaries in ZnO-Bi.sub.2 O.sub.3 Varistor", J. Amer. Ceram. Soc.-Disc. and Notes, 62(34), 221 (1979); D. R. Clarke, "Grain Boundary Segregation in a Commercial ZnO-based Varistor", J. Appl. Phys., 50(11), 6829-32 (1979); L. M. Levinson and H. R. Philipp. "The Physics of Metal Oxides Varistors", J. Appl. Phys. 46, 1332 (1975); D. R. Clarke, "The Microstructural Location of the Intergranular Metal-Oxide Phase in a Zinc Oxide Varistor", J. Appl. Phys. 49, 2407 (1978).] Because of the former location, it is thought to contribute to the large electrostatic barriers which form at the grain boundaries [Pike, Mat. Res. Soc. Proc., (5) 369 (1982)]. Other dopants, such as Co and Mn oxides, are used to enhance specific electrical properties such as increasing nonlinearity coefficients [Miyoshi, et al., Ad. Ceram., Vol. 1, 309-15 (1981)], which disclosure is being incorporated by reference herein. These dopants are generally reported to be homogeneously distributed in ZnO grains [P. Williams, D. L. Kirvanek, G. Thomas and M. Yodogawa, "Micro-structure-Property Relationships of Rare Earth-ZnO Varistors", J. Appl. Phys. 51(7), 3930-4 (1980); L. J. Bowen and F. J. Avella, "Microstructure, Electrical Properties, and Failure Prediction in Low Clamping Voltage Znc Oxide Varistors", J. Appl. Phys. 54(5), 2764-72 (1983)]. Doped ZnO varistors have been reported as showing average voltage drops per grain boundary in the range of 2-4 volts when operating in the nonlinear regime [J. T. C. van Kemednade and R. K. Eijnthoven, "Direct Determination of Barrier Voltage in ZnO Varistors", Ber. Dtsch. Keram. Ges. 55(6), 330 (1978); P. R. Emtage, "The Physics of Zinc Oxide Varistors", J. Appl. Phys. 48(10), 4372 (1977)] . Typical doped ZnO varistors are described in Wong, "Sintering and Varistor Characteristics of ZnO-Bi.sub.2 O.sub.3 Ceramics", J. Appl. Phys. 51(8), Aug. 1980, which disclosure is incorporated by reference herein.
Commercially available ZnO varistors are typically intended to operate in the nonlinear current-voltage regime with relatively low applied electric fields. They are prepared using conventional ceramic techniques which rely on mechanical mixing of oxide components for homogeneity and require high sintering temperatures (1000.degree.-1300.degree. C.) for densification. These varistors are thus limited to relatively low voltage applications due to compositional and microstructural heterogeneity and to large average grain sizes (&gt;3 .mu.M) caused by exaggerated grain growth during high temperature sintering. Low field strength &lt;5 kV/cm and .alpha. values in the 30-50 range typically result. Accordingly, the varistors so produced are not suitable for applications in which high field strength (&gt;5 kV/cm) is desirable. The highest field strength available in a custom designed varistor is in the range of 40-45 kV/cm and is believed to be unique to a single manufacturer.
U.S. Pat. No. 4,297,250 to Gupta, et al., which disclosure is being incorporated by reference herein, discloses the production of ZnO powders for use as variable (non-linear) resistors. The process involves mixing up to 98 mole% ZnO with up to 25 mole% (preferably 4-8%) of other metal oxides such as Bi.sub.2 O.sub.3, MnO.sub.2, CoO and the like, in an aqueous solution with an organic binder. The mixture is dried, pressed, sintered, then crushed to produce a powder.
U.S. Pat. No. 4,180,483 to Hoe, et al., which disclosure is being incorporated by reference herein, discloses a zinc oxide powder for use in non-linear resistors. The powder is produced by adding a mixed oxide glass powder (containing Bi.sub.2 O.sub.3 and other metal oxides) to ZnO powder, then heating and pressing the result to form a ceramic body. This body is then annealed to produce the final product.
U.S. Pat. No. 4,243,622 to Kresge, which disclosure is being incorporated by reference herein, discloses the production of zinc oxide varistors through mixing and blending zinc oxide and other metal oxides, formed by sintering. The resulting composition is primarily zinc oxide, with small amounts of other oxides including Bi.sub.2 O.sub.3, CoO and MnO.sub.2.
U.S. Pat. No. 4,285,839 to Wong, which disclosure is being incorporated by reference herein, discloses the production of varistors and varistor powders primarily of zinc oxide by predoping the zinc oxide with aluminum nitrate in solution. The zinc oxide powder is added to the AlNO.sub.3 solution, then heated, followed by the addition of other metal oxides. The final powder is sintered at 900.degree.-1000.degree. C.
U.S. Pat. No. 4,405,508 to Eckel, which disclosure is being incorporated by reference herein, discloses the production of zinc oxide varistor materials by combining zinc oxide, bismuth oxide and antimony oxide to form a Bi.sub.6 Zn.sub.4 Sb.sub.2 O.sub.18 pyrochlore, then grinding this compound while adding other oxides such as cobalt oxide or manganese oxide. The resulting materials are pressed and sintered at 1150.degree. C. to obtain the varistor.
Lauf, et al., have recently reported using chemical preparation techniques to prepare individual oxide compositions of ZnO-based varistors and using the individual components to prepare varistors by conventional oxide mixing techniques with densification done by hot pressing. Lauf, et al., "Fabrication of High-Field Zinc Oxide Varistors by Sol-Gel Processing," Am. Ceram. Soc. Bull., 63(2), 270-81 (1984). By subsequent heat treatments in oxidizing atmospheres, they have produced materials with 3-6 .mu.M grain size and field strengths of 10 kV/cm at 10.sup.-4 A/cm.sup.2 and .alpha.=30(10.sup.-5 to 10.sup.-4 A/cm.sup.2). Assuming .alpha.=30 from 10.sup.-5 to 5 A/cm.sup.2, a field strength of 14.3 kV/cm would be measured at 5 A/cm.sup.2.
In Wong, U.S. Pat. No. 4,142,996, which disclosure is being incorporated by reference herein, a fused salt method is used instead of mechanical mixing in an effort to obtain homogeneous varistor powders. Zinc and other metal nitrates were heated to form a hydrated melt which is further heated to dehydrate the melt, followed by still further heating at 600.degree.-800.degree. C. to convert the salts to oxides. The oxides are then sintered at 1300.degree. C. producing varistors with breakdown fields of 1.6-2.3 kV/cm.
A brief comparison of these processes and that of the invention as described below is summarized in the following chart:
______________________________________ Sintering Average Powder Tem- Grain Breakdown Prep perature Size Voltage ______________________________________ Cited Mixed oxide 1000-1300.degree. C. &gt;5 .mu.M &lt;5 kV/cm Patents Fused salts In- Co-precip- 675-740.degree. C. &lt;1 .mu.M 30-100 kV/cm vention itiation ______________________________________